SpamAssassin Rule: FORGED_MUA_OUTLOOK
Standard description: Forged mail pretending to be from MS Outlook
The message headers include those generated by Microsoft Outlook, such as X-Mailer, but the syntax is not the same as created by genuine Microsoft software.
This suggests that the message was created by a mailout program rather than a user with a Microsoft email client.
The default scores for this rule can be found in the online list of tests.